Array Texture

An Array Texture is a Texture where each mipmap contains level an array of images of the same size. Array textures may have Mipmaps, but each mipmap in the texture has the same number of levels.

The domain of the images in the array depend on the array texture's specific type. Array textures come in 1D and 2D types, with cubemap arrays available on some hardware.

The high cost of texture switches resulted in a widespread preference for texture atlases (a single texture which stores data for multiple objects). This resulted in fewer texture switching, and thus less overhead. Array textures are an alternative to atlases, as they can store multiple layers of texture data for objects without some of the problems of atlases.

Array texture are not usable from the fixed function pipeline; you must use a Shader to access them.

Contents

Terminology

Each mipmap level of an array texture is a series of images. Each image within a mipmap is called a "layer".

Creation and Management

1D array textures are created by binding a newly-created texture object to GL_TEXTURE_1D_ARRAY​, then creating storage for one or more mipmaps of the texture. This is done by using the "2D" image functions; the "height​" parameter sets the number of layers in the array texture.

Every row of pixel data in the "2D" array of pixels is considered a separate 1D layer.

2D array textures are created similarly; bind a newly-created texture object to GL_TEXTURE_2D_ARRAY​, then use the "3D" image functions to allocate storage. The depth​ parameter sets the number of layers in the array.

Each 2D row/column sequence of pixel data in the "3D" array of pixels is considered a separate 2D layer.

GLuint texture =0;
GLsizei width =2;
GLsizei height =2;
GLsizei layerCount =2;
GLsizei mipLevelCount =1;//Read you texels here. In the current example, we have 2*2*2 = 8 texels, with each texel being 4 GLubytes.
GLubyte texels[32]={//Texels for first image.0,0,0,255,255,0,0,255,0,255,0,255,0,0,255,255,//Texels for second image.255,255,255,255,255,255,0,255,0,255,255,255,255,0,255,255,};
glGenTextures(1,&texture);
glBindTexture(GL_TEXTURE_2D_ARRAY,texture);//Allocate the storage.
glTexStorage3D(GL_TEXTURE_2D_ARRAY, mipLevelCount, GL_RGBA8, width, height, layerCount);//Upload pixel data.//The first 0 refers to the mipmap level (level 0, since there's only 1)//The following 2 zeroes refers to the x and y offsets in case you only want to specify a subrectangle.//The final 0 refers to the layer index offset (we start from index 0 and have 2 levels).//Altogether you can specify a 3D box subset of the overall texture, but only one mip level at a time.
glTexSubImage3D(GL_TEXTURE_2D_ARRAY,0,0,0,0, width, height, layerCount, GL_RGBA, GL_UNSIGNED_BYTE, texels);//Always set reasonable texture parameters
glTexParameteri(GL_TEXTURE_2D_ARRAY,GL_TEXTURE_MIN_FILTER,GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D_ARRAY,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D_ARRAY,GL_TEXTURE_WRAP_S,GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D_ARRAY,GL_TEXTURE_WRAP_T,GL_CLAMP_TO_EDGE);

Note that, unlike for 3D Textures, each mipmap uses the same number of layers. So if you're allocating 3 mipmaps of a 2D array texture, it would look like this:

Access in shaders

Texture arrays have separate sampler types: sampler1DArray​ and sampler2DArray​. When accessing them within a shader, you use one extra texture coordinate. So sampler1DArray​ would use a 2D texture coordinate, while sampler2DArray​ would use a 3D texture coordinate.

The last coordinate is the layer number to access. For floating-point texture coordinates (when not using texture functions like texelFetch​), the floating-point layer is rounded then clamped to compute the integer layer index by the following function:

actual_layer = max(0, min(d​ - 1, floor(layer​ + 0.5)) )

Here, d​ is the number of layers in the texture, and layer​ is the floating-point layer from the texture coordinate. For example, to sample at s = 0.4, t = 0.6, for layer 2, use vec3(0.4, 0.6, 2).

Cubemap arrays

Limitations

Array texture sizes are normally limited to the usual GL_MAX_TEXTURE_SIZE​ limitation. However, the dimension that represents the number of array levels is limited by GL_MAX_ARRAY_TEXTURE_LAYERS​, which is generally much smaller. OpenGL 4.5 requires that the limit be at least 2048, while OpenGL 3.3 requires it be at least 256.